My understanding of how this works is this:
You need to pay attention to what is in the exact center of your 3D Window.
If there is an element, such as a Wall then the 3D Model is rotated around that point on the surface of that Wall which is in the center of the 3D Window.
In all other cases the model is rotated around the center of the 3d bounding box of the model.
You can check this.
Pan your model so a Wall on its left end is in the middle of the screen. Rotate, it will be rotated so that point of the surface of the Wall stay in place.
But pan the model to the right a little so there is no part of the building in the middle of the screen. The model will be rotated on the right, around the center of its 3d bounding box.
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